1. Field of the Invention
The present invention relates to a patterned thin film containing a plurality of stacked sublayers and a method of forming the same.
2. Description of the Related Art
Frame plating is one of the methods of forming a thin film that is patterned (referred to as patterned thin film in the present patent application), such as the method disclosed in Published Examined Japanese Patent Application Showa 56-36706 (1981). To perform the frame plating disclosed, an electrode film is formed on a substrate, for example, and a resist layer is formed on the electrode film. The resist layer is patterned by photolithography to form a frame to be used for plating. Electroplating is then performed through the use of the frame with the electrode film already formed as an electrode and a seed layer, so as to form a patterned thin film.
A patterned thin film formed by frame plating may be utilized in microdevices, for example. Such microdevices include a thin-film inductor, a thin-film magnetic head, a semiconductor device, a sensor incorporating thin films, and an actuator incorporating thin films. In such applications a patterned thin film that is small in width and great in thickness may be required in some cases in order to increase the integration of a microdevice, for example.
Consideration will now be given to a case in which frame plating is utilized to form the above-mentioned patterned thin film that is small in width and great in thickness. If a single-layer frame is used to form the patterned thin film in this case, it is required to make a frame that has a great thickness and a groove with a reduced width. However, it is very difficult to make such a frame with accuracy.
To solve this problem, a method is disclosed in Published Unexamined Japanese Patent Application Heisei 8-330736 (1996) to form a patterned thin film by stacking two or more plating layers. In this method an electrode film is first formed on a substrate, for example, and a first resist layer is formed on the electrode layer. The first resist layer is patterned by photolithography to form a first frame. Next, an electric current is fed to the electrode film to perform electroplating through the use of the first frame to form a first plating layer. The first frame is then hard-baked, without peeling off the first frame. Next, a second resist layer is formed on the first frame and the first plating layer. The second resist layer is patterned by photolithography to form a second frame. A current is fed to the electrode film used for making the first plating layer, so as to perform electroplating through the use of the second frame to form a second plating layer. The second plating layer is stacked on the first plating layer. Next, the first and second frames are peeled off through the use of an organic solvent. Finally, portions of the electrode film except the portion below the first plating layer are removed by wet etching or dry etching.
Alternatively, another plating layer or more may be stacked on the second plating layer through a step similar to the step of forming the second plating layer on the first plating layer. It is acceptable that the second plating layer and the plating layers above are in contact with at least part of the plating layer located directly below. It is also acceptable that two overlapping plating layers have different patterns.
In the above-described method of forming the patterned thin film, if each of the resist layers is made through the use of a liquid resist, after the liquid resist is applied to form a coating film, heat processing, that is, prebaking of the coating film is required to evaporate the solvent contained in the coating film and to enhance the intimate contact between the coating film and the layer below.
However, it has been found that the following problem arises when each of the resist layers is made through the use of a liquid resist. When prebaking is performed to form the second resist layer and the resist layers above, the coating film provided for the resist layer to be formed, or the resist layer (the frame) below the coating layer form foams. As a result, it is difficult to make the resist layer to be formed without defects.
The above-mentioned Published Unexamined Japanese Patent Application Heisei 8-330736 discloses the method of forming the first and second resist layers through the use of a dry film photoresist. The abovementioned foaming when prebaking is performed will not occur if the resist layers are made of the dry film photoresist. However, the resist layers made of the dry film photoresist exhibit a lower resolution than resist layers made of a liquid resist. It is therefore difficult to form a fine patterned thin film when the resist layers are made of the dry film photoresist.